1. Field of the Invention
This invention relates to a mixer circuit for use in a converter (frequency converting circuit), a measuring instrument, etc.
2. Related Background Art
Through a recent attempt to rapidly developing an information network system, demand for a satellite communication system has been also rapidly increased, and a frequency band has been shifted to high frequency. In this type of satellite communication system, a down converter for converting a high-frequency signal to a low-frequency signal is required, and a mixer circuit which is applied to the down converter has been also increasingly demanded. There have been conventionally utilized various types of mixer circuits, each of which uses any one of a diode, a bipolar transistor, a FET (field effect transistor), etc. as an active device thereof. An normal MESFET (Metal-semiconductor field effect transistor) or a HEMT (high electron mobility transistor) has been generally used.
It is pointed out as one of most critical problems in design of a mixer circuit using a FET, that is, a FET mixer circuit, that a gate voltage of the FET is greatly fluctuated because an LO (local oscillation) signal is a large-amplitude signal. The variation of the gate voltage induces variation of mutual conductance g.sub.m of the FET, so that a gate/source capacitance C.sub.gs, and thus input impedance is also varied. That is, viewing from an input side for RF (radio frequency) signals, the FET mixer circuit operates such that the input impedance of the circuit greatly varies as a time function in accordance with fluctuation of the LO signal. Therefore, design of the FET mixer circuit is difficult, and a stable operation thereof is hardly expected.